The government's earthquake research committee said Sunday an unknown active fault may have caused a magnitude 6.3 earthquake that jolted western Japan early Saturday.
The active fault is believed to extend about 10 km from north to south, according to the committee.
"There are many as-yet-unrecognized active faults," Yoshimori Honkura, the committee chief, told a press conference following its extraordinary meeting, warning that a magnitude 6-level quake could happen anywhere in Japan.
The committee members also shared a view that the quake on Saturday was associated somehow with the 1995 Great Hanshin Earthquake.
It also said 390 aftershocks were recorded in the 25 hours since the main tremor, and urged the public to stay alert to possible aftershocks for around one week.
The largest aftershock, with a magnitude of 3.8, occurred at 5:41 a.m. Saturday, eight minutes after the main tremor, according to the committee.
The main quake on Saturday registered lower 6 on Japan's seismic intensity scale of 7 on Awaji Island in Hyogo Prefecture, the highest intensity recorded in western Japan since the 1995 disaster.
Twenty-five people were injured, and around 1,930 houses were damaged, mainly on Awaji Island, by Saturday's earthquake.